The rule searches for where the XQuery function are used in place of Algebricks builtin function.
The combination of the boolean XQuery function and the XQuery equivalent function are replaced with
the Algebricks builtin function .

Before
plan__parent
%OPERATOR( $v1 : xquery_expression( \@input_expression ) )
plan__child
where xquery_expression has a known equivalent in Algebricks,
such as conditional expressions and a check for null.
The expression may include the boolean function to ensure only
a true or false result.
After
plan__parent
%OPERATOR( $v1 : algebricks_function( \@input_expression ) )
plan__child